Earth flow (Dosekiryu) at Mt. Myoko on 18th May, 1978

1) At about 06:20 o'clock on 18th May, 1978, the slope area of about 1.3ha, located at 1,600m above the sea level, was broken and a mass of about 200,000m^3 were slid into the valley of river Shirotagiri-gawa. The slid material was converted to the earth flow (Dosekiryu), which flowed down along the valley of river Shirotagiri-gawa about 1,150m in vertical height and about 7.5Km in horizontal distance. This was the first time of earth flow. 2) The most violent disaster occurred in the residential area of the Shin-akakura hot spring of Myokokogen-machi, located on the left bank of the river Shirotagiri-gawa, where 11 houses were destroyed and 11 persons were dead (Fig.1). 3) As the slid material fell down a short distance, it passed through the flat step composed of hard lava playing a role of the chute at the base of the slope, and then the earth flow attained great velocity because of the steep slope of about 60°. The rock fragments in part were lanched into the air from the step. It is hypothesized that the sheet of debris trapped air between itself and the river floor to from an air cushion on which the debris slid freely. At the early stage of the earth flow, it is assumed that air was entrained in the falling debris, so that the slid material might behave like a liquefied mass. 4) On the upper steeper slopes from the initial slid area to the point about 950m above the sea level, the effect of the earth flow were largely erosive, but on the gentle slopes (1/10.4 on average) on the down stream below 950m, it became to deposit material, where the total volume of newly deposited material was estimated to be about 410,000m^3. As the initial volume of earth flow was assumed to be about185.000m^3, in the course of flowing down of 3.5Km along the Minami jigokudani valley with an average slope 1/4, debris of about 200,000m^3 had to be supplied mainly by its erosion of the talus deposits on the southern flank and also by gouging out the rock and soil on river floor. 5) In the A-area of Shin-akakura, the earth flow ran straightly over the left-side embankment, spreading as it went, to form eventually flat, and it covered the area of 76,100m^2 with lobe-shaped deposits of about 87,950m^3. At the edge and periphery of the debris, the flow was retarded and was stopped by the houses and buildings. 6) In the C-area of Shin-akakura, the flow was spreaed and covered the area of about 68,700m^2 with the debris deposits of about 111,050m^3. The earth flows successively ran down as 12 individual waves. 7) The earth flow reached a maximum velocity of approximately 20m/sec as it flowed down the Minamijigokudani valley. In the stream near the C-area of Shin-akakura, 19 individual waves were observed. The height of wave front was 5 to 6m at first, then lowering to 1 to 0.5m step by step. 8) Most effective causes for over-flooding at the A, B and C areas of Shin-akakura are ascribed probably to the leveling up of the river floor due to the construction of the embankment for change of the stream from original natural course to the artificially excavated river Fukazawa-gawa. 9) The second slide, of about 25,000m^3 in volume, took place at 13:40 o'clock on the same day, which changed to the second time of earth flow. It flowed down without over-flooding and excavated the bank of national railway near the river Sekikawa, where 3 persons were dead. The velocity at this time was estimated to be about 14m/sec